1. Field of the Invention
The present invention relates to optical collimators, and more particularly, to collimators for precise alignment of optical paths and method of making same.
2. Background Art
Optical collimators have been widely used in optical fiber communications networks, systems and devices to collimate light transmitted by optical fibers, in order to form substantially parallel light beams in free space in various types of optical devices, especially non-integrated optical devices, including, for example, optical switches, isolators, attenuators, beam splitters and beam combiners. Collimators perform an important function of preventing excessive insertion loss due to dispersion of light beams in free space in these optical devices.
FIG. 1 shows a side sectional view of a conventional collimator with a graded index (GRIN) lens for collimating a light beam transmitted by an optical fiber. In FIG. 1, a section of optical fiber 2 has a terminal 4 connected to a capillary 6, which has an index of refraction n1. The capillary 6 typically has a cylindrical shape with a center axis 8. In a conventional collimator with a GRIN lens 12, the capillary 6 typically has an end surface 10 which is slanted slightly off the normal to the center axis 8, in order to prevent total reflection of an incoming light beam received from the optical fiber 2 back to the optical fiber. The GRIN lens 12 also typically has a cylindrical shape centered about the axis 8 and an end surface 14 opposite the end surface 10 of the capillary 6.
In a conventional collimator, the end surface 14 of the GRIN lens 12 is also slanted slightly off the normal to the center axis 8. A gap 16 is typically provided between the end surface 10 of the capillary 6 and the end surface 14 of the GRIN lens 12. Both of these end surfaces may be slanted at an angle of about 8xc2x0 off the normal to the center axis 8, for example, and are made to be substantially parallel to each other. In a conventional collimator, the gap 16 is typically filled with a gas such as air, which has an index of refraction n2, while the GRIN lens 12 has an index of refraction n3. In a conventional collimator equipped with a GRIN lens, the index of refraction n3 of the GRIN lens is typically different from the index of refraction n1 of the capillary 6 because they are made of different materials. Because of the differences between the indices of refraction n1 and n2 at the end surface 10 of the capillary 6 and between the indices of refraction n2 and n3 at the end surface 14 of the GRIN lens 12, an incoming light beam that enters the capillary 6 along the center axis 8 typically deviates from the center axis 8 at an angular deviation xcex1 with respect to the center axis when the light beam exits the GRIN lens 12.
Both the capillary 6 and the GRIN lens 12 are enclosed by a cylindrical metal sleeve 18, which may be made of gold plated stainless steel, for example, with a inner cylindrical surface 20 and an outer cylindrical surface 22 centered about the center axis 8. One or more concentric cylindrical layers of protective materials may be provided between the inner surface 20 of the metal sleeve 18 and side walls of the capillary 6 and the GRIN lens 12, depending upon the construction of the collimator. Because of process variations in the manufacturing of a conventional collimator such as the one shown in FIG. 1, slight variations in the angles of the slanted end surfaces 10 and 14 of the capillary 6 and the GRIN lens 12 may result in unpredictability of the angular deviation xcex1 of the output light beam 24 with respect to the center axis 8 of the collimator.
Furthermore, because the cylindrical collimator may be rotated unpredictably when it is assembled to an optical device, the direction of the output light beam 24 emanating from the collimator is even more unpredictable. In addition, the incoming light beam that enters the capillary 6 of the collimator from the optical fiber 2 may not be perfectly aligned with the center axis 8 of the collimator, thereby causing a translational offset xcex94 in addition to the angular deviation xcex1 with respect to the center axis. Other process variations such as tolerance of GRIN lens specifications may also contribute to the unpredictability of the direction of the output light beam emanating from the collimator.
When conventional collimators such as the one shown in FIG. 1 and described above are assembled to an optical device in which at least some of the light beams need to travel in free space between the collimators, alignment of light beams between different collimators can be very difficult and time-consuming. Translational offset and angular deviation of light beams emanating from collimators usually exist and are usually unpredictable regardless of the types of lenses used, such as conventional GRIN lenses, ball lenses or C lenses, even if they are manufactured with tight specifications. An output light beam emanating from a conventional collimator typically has a very small spot size with a diameter as little as 200 xcexcm, for example. Therefore, even a slight offset or deviation may cause misalignment of optical paths between collimators in an optical device.
FIG. 2 illustrates a simplified sectional view of a typical non-integrated optical device, which may be an optical switch, an isolator, an attenuator, a beam splitter or a beam combiner, for example, with two collimators 26 and 28 serving as two optical ports of the device. An optical device element 30 may be movably positioned between the collimators 26 and 28. The optical device element 30 may be a prism or mirror if the optical device is an optical switch, or an attenuator or isolator element if the optical device is an isolator or attenuator, for example. The optical device typically has a metal packaging 32 for enclosing the optical element 30. In FIG. 2, the collimators 26 and 28 are fixed to sidewalls 34 and 36 of the metal packaging 32, respectively. The collimators may be fixed to the side walls of the packaging in various conventional manners, for example, by using epoxy gluing, tin soldering or laser welding techniques.
In a typical non-integrated optical device, such as a multi-port optical switch, the collimators 26 and 28 may be placed far from each other, with a distance measured in centimeters. The distances between different collimators in a multi-port optical device make optical alignment between the collimators even more difficult. A light beam travelling in free space within an optical device typically has a very narrow beam width that produces a very small light spot with a Gaussian distribution, with negligibly low light levels outside the spot area. A receiving collimator may not collect enough optical energy even if it is slightly out of alignment with the optical path of the light beam emanating from a transmitting collimator, thereby resulting in a huge loss of optical signals.
Alignment of collimators may be achieved in a typical non-integrated optical device by trial and error, although the labor intensiveness of such an approach is self-evident. The problem of alignment using the trial-and-error approach is exacerbated further in a multi-port optical device such as an Mxc3x97N optical switch, which requires precise alignment of many different combinations of optical paths between the collimators through different combinations of optical switching elements, such as tilted mirrors or prisms. The problem associated with optical alignment is a major factor for the high cost and slow production rate of typical non-integrated multi-port optical switches at the present time.
Furthermore, when the collimators are fixed to the packaging of a typical optical device, whether by using epoxy glue, tin solder or laser welding, an assembly technician may need to continually adjust the orientation of each of the collimators while gluing, soldering or welding the collimator to the optical device packaging. Process variations in conventional gluing or soldering techniques may also ultimately affect the optical alignment of the collimators. For example, epoxy glue typically takes several hours to cure, and during the curing process, the epoxy glue may deform slightly to cause misorientation of the collimators after the application of the epoxy glue. If the collimators are soldered to the packaging of an optical device using tin solder, for example, the alignment of the collimators may be adversely affected by the heating, cooling and solidifying of solders around the metal sleeves of the collimators.
In order to facilitate the alignment of collimators in an optical device to compensate for the effects of translational offsets and angular deviations of light paths, optimization techniques have been proposed for collimator alignment using a laser source, a photodetector and optimization software on a computer. For example, a laser source (not shown) may be connected to an input optical fiber 38 which is connected to the first collimator 26 in FIG. 2, while a photodetector (not shown) may be connected to an output optical fiber 40 which is connected to the second collimator 28.
The laser source provides a light beam which emanates from the collimator 26 into the free space along an optical path 42, which has a translational offset and an angular deviation with respect to the center axis 43 of the first collimator 26. The first collimator 26 may be initially fixed to the sidewall 34 of the optical device packaging 32, while the second collimator 28 is initially movable such that it can reach the optical path 42 along which the light beam emanating from the first collimator 26 travels inside the optical device, to allow the photodetector which is connected to the second collimator 28 to detect the light beam. If the second collimator 28 can receive some optical energy from the light beam 42, the computer running the optimization software may at least theoretically be able to find an optimal position and orientation for the second collimator 28 to receive the light beam 42 emanating from the first collimator 26.
In practice, however, one needs to be lucky to find an initial position and orientation for the second collimator 28 to allow it to receive at least a detectable amount of optical energy in the first place, because the collimated light beam travelling along the optical path 42 may produce only a small light spot, for example, with a diameter as small as 200 xcexcm. The light spot produced by the light beam along the optical path 42 typically has a Gaussian distribution with very low power densities outside the spot area. If the photodetector that is connected to the second collimator 28 is unable to detect the light beam in the first place, it would be a futile attempt for the computer optimization software to find the optimal position and orientation for the second collimator 28 to receive the light beam. Therefore, even with the aid of computer optimization software for the alignment of optical paths between different collimators in an optical device, one still needs to adjust the locations and orientations of the collimators by trial and error to obtain at least a coarse alignment before the computer can establish initial data points to run the optimization software to find appropriate alignment solutions.
The labor intensiveness and low productivity resulting from conventional optical alignment techniques are usually major contributing factors for high costs of manufacturing non-integrated optical devices. Furthermore, in multi-port optical devices, such as Mxc3x97N optical switches, precise alignment of optical paths need be achieved for every switchable combination of every pair of input and output collimators. With manual adjustments of collimators to obtain precise optical alignment, adequate spacings need be provided between the collimators and optical elements, such as prisms or mirrors in case of an optical switch, to allow for such adjustments of the collimators. The need for manual adjustments of collimators using conventional optical alignment techniques makes it difficult to produce a compact non-integrated optical device with close spacings between collimators and optical elements.
The present invention provides a collimator for ready fitting to an optical device with precise alignment of the output optical path. In an embodiment, the collimator generally comprises:
a collimator lens; and
a sleeve enclosing the collimator lens, the sleeve having an inner cylindrical surface centered about a collimator axis and an outer cylindrical surface that is concentric with the optical path which has a translational offset or an angular deviation with respect to the collimator axis.
The present invention also provides a method of making a collimator having a lens centered about a collimator axis and enclosed by an outer sleeve for precise alignment of an optical path emanating from the collimator, to obviate the need for adjusting the collimator to compensate for a translational offset or an angular deviation of the optical path with respect to the collimator axis. In an embodiment, the method generally comprises the step of removing a portion of the outer sleeve according to the offset and the deviation, to form an outer cylindrical surface centered about an axis that coincides with the optical path.
Advantageously, the collimator manufactured according to embodiments of the present invention with an outer cylindrical surface centered about an axis that coincides with the optical path emanating from the collimator can be readily fitted to an optical device without need for further adjustment of the position or orientation of the collimator in the optical device to achieve precise optical alignment. Furthermore, the time and labor cost required for manufacturing various types of optical devices can be greatly reduced with the implementation of collimators manufactured according to embodiments of the present invention, thereby significantly increasing the productivity in optical device manufacturing.